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Heart rate is a term used to describe the frequency of the cardiac cycle. Usually it is calculated as the number of contractions (heart beats) of the heart in one minute and expressed as "beats per minute" (bpm). See "Heart" for information on embryofetal heart rates. Psychologists monitor heart rate for a number of reasons, eg in anxiety and sleep research

When resting, the adult human heart beats at about 70 bpm (males) and 75 bpm (females), but this rate varies between people. However, the reference range is nominally between 60 bpm (if less termed bradycardia) and 100 bpm (if greater, termed tachycardia). Resting heart rates can be significantly lower in athletes, and significantly higher in the obese.

The pulse is the most straightforward way of measuring the heart rate, but it can be deceptive when some strokes do not lead to much cardiac output. In these cases (as happens in some arrhythmias), the heart rate may be considerably higher than the pulse.

An electrocardiograph, or ECG, is the most precise method of heart rate measurement. Continuous electrocardiographic monitoring of the heart rate is routinely done in many clinical settings, especially in critical care medicine. Commercially available heart rate monitors are also available, consisting of a chest strap with electrodes. The signal is transmitted to a wrist receiver for display. Heart rate monitors allow accurate measurements to be taken continuously and can be used during exercise when manual measurement would be difficult or impossible (such as when the hands are being used).

It is also possible to measure heart rate acoustically, by listening to the sounds the heart makes while beating. These sounds can be amplified through the use of a stethoscope.

Heart rate variability (HRV) is the variation of beat-to-beat intervals. A healthy heart has a large HRV, while decreased or absent variability may indicate cardiac disease. HRV also decreases with exercise-induced tachycardia.

One aspect of heart rate variability can be used as a measurement of fitness, specifically the speed at which one's heart rate drops upon termination of vigorous exercise. The speed at which a person's heart rate returns to resting is faster for a fit person than an unfit person. A drop of 20 beats in a minute is typical for a healthy person.

Resting Heart Rate is a term used to describe a person's heart rate when they are not performing any activities. It is best measured first thing in the morning, before even getting out of bed.

A person's resting heart rate typically rises with age. Normal rates are in the range from 60 to 80. Fit people generally have lower HRrest, so it is often used as a measure of fitness. Elite endurance athletes may have resting heart rate less than 30.

Measuring HRrest every day is a good way of detecting possible illness, as the RHR will be elevated by 8–10 beats if the immune system is attempting to fight something.

Maximum Heart Rate (also called MHR, or HRmax) is the maximum heart rate that a person can achieve during maximal physical exertion. Research indicates it is most closely linked to a person's age; a person's HRmax will decline as they age. Some research indicates the speed at which it declines over time is related to fitness—the more fit a person is, the slower it declines as they age.

HRmax is used frequently in the fitness industry, specifically during the calculation of Target Heart Rate when prescribing a fitness regimen. The quickest way to determine your MHR is to subtract your age from 220.

Heart rate is increased by physiological activity and by certain psychological influences (e.g., fear). On the other hand, heart rate is also
strongly correlated with the body temperature. In absence of influences above, heart rate and bodytemperature are linearly connected.
In particular, the change of 10 beats per minute in heart rate corresponds to the change of 1 degree Celsius in body temperature. This is very useful in determining the body temperature when thermometers are not available, provided that person knows his or her normal body temperature and heart rate.

The recovery heart rate is one that is taken several minutes after exercise. It is taken anywhere between 2–10 minutes after exercise. It is taken for 15 seconds, and is multiplied by four in order to calculate beats per minute (BPM). The goal is to not exceed 120 BPM.

The most accurate way of measuring HRmax for an individual is via a maximal exercise test. In such a test, the subject exercises while being monitored by an electrocardiogram (ECG). During the test, the intensity of exercise is periodically increased (if a treadmill is being used, through increase in speed or slope of the treadmill) until the subject can no longer continue, or until certain changes in heart function are detected in the ECG (at which point the subject is directed to stop). Typical durations of such a test range from 10 to 20 minutes.

Conducting an accurate maximal exercise test requires expensive equipment, and should only be performed in the presence of medical staff due to risks associated with high heart rates. Instead, people typically use predictive formulae to estimate their individual Maximum Heart Rate. The most common formula encountered is:

HRmax = 220 − age (can vary)

This is often attributed to various sources, including "Fox and Haskell". While the most common (and easy to remember and calculate), this particular formula is not considered by some to be a good predictor of HRmax.

A 2003 study [1] of 43 different formulae for HRmax (including the one above) concluded the following:

1) No "acceptable" formula currently existed, (they used the term "acceptable" to mean acceptable for both prediction of VO2max, and prescription of exercise training HR ranges)

2) The most accurate formula of those examined was:

HRmax = 205.8 − (0.685 * Age)

This was found to have a standard of error, that although large (6.4 BPM), still deemed to be acceptable, for the use of prescribing exercise training HR ranges.

Target Heart Rate (THR), or Training Heart Rate, is a desired range of heart rate reached during aerobic exercise which enables one's heart and lungs to receive the most benefit from a workout. This theoretical range varies based on one's physical condition, age, and previous training. Below are three ways to calculate one's Target Heart Rate. In each of these methods, there is an element called "Intensity" which is expressed as a percentage. THR can be calculated by using a range of 50%–85% intensity.

Heart Rate Reserve (HRR) is a term used to describe the difference between a person's Resting Heart Rate, and Maximum Heart Rate. Some methods of measurement of exercise intensity measure percentage of Heart Rate Reserve. Additionally, as a person becomes fit, as their HRrest will drop, the Heart Rate Reserve will increase.

Bradycardia is defined as a heart rate less than 60 beats per minute although it is seldom symptomatic until below 50 bpm. Trained athletes tend to have slow resting heart rates, and resting bradycardia in athletes should not be considered abnormal if the individual has no symptoms associated with it.